Sine rule

Fig. 1. Sine rule: sinα:sinβ:sinγ=a:b:c

In trigonometry, the sine rule (also known as Law of Sines) relates in a triangle the sines of the three angles and the lengths of their opposite sides,

${\frac {a}{\sin \alpha }}={\frac {b}{\sin \beta }}={\frac {c}{\sin {\gamma }}}=d,$

where d is the diameter of the circle circumscribing the triangle and the angles and the lengths of the sides are defined in Fig. 1. From this follows that the ratio of the sines of the angles of a triangle is equal to the ratio of the lengths of the opposite sides.

Proof

The easiest proof is purely geometric, not algebraic.

Lemma

Fig. 2. The angles α and α' share the chord a. The center of the circle is at C and its diameter is d.

In Fig. 2 the arbitrary angle α' satisfies,

$\sin \alpha '={\frac {a}{d}},$

where d is the diameter of the circle and a is the chord opposite α'. This follows because the two angles, α and α', in Fig. 2 share a segment of the circle (have the chord a in common). By a well-known theorem of plane geometry it follows that the two angles are equal. The angle α, having the diameter of the circle d as one of its sides, has as opposite angle a right angle. Hence sin(α) = a/d, the length of chord a divided by the diameter d. It follows that the angle α' has the same sine as α.

Fig. 3

Proof of sine rule

From the lemma follows that the angles in Fig. 3 are

$\sin \alpha ={\frac {a}{d}},\quad \sin \beta ={\frac {b}{d}},\quad \sin \alpha ={\frac {c}{d}},$

where d is the diameter of the circle. From this result the sine rule follows.

External link

Life lecture on Sine rule